Serial communications/interconnect protocols provide efficient mechanisms to communicate between different devices. These protocols can include standards defining signal properties, timing and state changes required for compatibility with the protocol. One serial communications protocol is the U[niversal]S[erial]B[us] protocol.
USB has been widely adopted in the electronics industry, wherein the USB 3 protocol enables a data rate of at least five Gigabits per second (5 Gbps), thus offering significant improvements in speed over USB 2.0 as well as significant power savings. USB 3 can be used in many different devices including, but not limited to, desktop computers, laptops, tablets, external hard drives, printers, cell phones and smart phones.
In this context, high-speed USB interfaces utilize a sideband of communication for managing signal initiation and low power management on the bus on a link between two ports. This sideband is referred to as Low Frequency Periodic Signalling (LFPS). LFPS employs a predetermined frequency range to communicate the initialization and power management information. For example, USB 3 utilizes LFPS whereas the previous two USB versions (=USB 1 and USB 2) do not utilize LFPS.
To ensure the proper operation of a high-speed interface using the USB 3 specification, a receiver must correctly detect high-speed data rates. Additionally, to reduce the cost of power management, the receiver may include a LFPS detector for detecting low-speed LFPS signals with a data rate of ten MHz to fifty MHz in a low-power USB 3.0 link.
A passive galvanic cable (inter)connection between a USB host and a USB device is limited to approximately 1.5 m.
A remote host device based on the USB 3 standard can be connected over a fiber to a USB root port; however, the USB standard based Low Frequency Periodic Signalling (LFPS) is not directly suitable for an electro-to-optical transmitter.
An active galvanic cable connection with repeaters based on the USB 3.1 standard requires complex implementations of the remote host device and the USB 3.1 repeaters due to full support of the protocol level.
The USB 3.1 standard defines the electrical idle (EI) state and two types of signalling for the communication between two USB 3.1 enabled devices. The first signalling type is the Low Frequency Periodic Signalling (LFPS), and the second type is the SuperSpeed (SS) signalling or enhanced SuperSpeed (eSS) signalling.
The LFPS together in combination with the electrical idle (EI) state creates an LFPS sequence or LFPS based PWM (pulse-width modulation) signalling (LBPS). The LBPS provides the basis for an LFPS based PWM message (LBPM).
The electrical idle (EI) state is defined as zero differential input voltage Vindiff. While in the electrical signalling domain such a third level can be easily transmitted (differential positive, differential negative, and zero differential input voltage), typical optical data transmission systems can usually transport only two signalling states: optical “0” and optical “1”.
Due to this electrical idle (EI) state, an LFPS sequence is not suitable for the direct transmission over an optical link. In order to transmit an LFPS sequence over an optical link it has to be translated first to a suitable data format.
In contrast, the SS/eSS signalling uses a D[irect]C[urrent]-balanced, non-return-to-zero (NRZ) line code, which is well suitable for a direct transmission over an optical link.